U.S. Pat. No. 4,551,910 discloses a method of the kind mentioned in the opening paragraph, in which the oxidation treatment is carried out initially under dry conditions and then under wet conditions. During the dry oxidation in a dry gas mixture containing hydrochloric acid, the wafer is heated to a temperature of 1000.degree. C., while during the wet oxidation in steam it is heated to a temperature of 920.degree. C. The dry oxidation is carried out until a layer of silicon oxide has formed having a thickness of about 50 nm, while the subsequent wet oxidation is carried out until the layer of silicon oxide has reached a desired thickness. As an oxidation mask a layer of silicon nitride is used, which is formed in part by thermal nitridation of the silicon and which is provided for the remaining part at a low pressure by deposition from the gaseous phase by means of an LPCVD process.
When, as described, the oxidation is initially carried out under dry conditions, the formation of defects in the silicon during the subsequent wet oxidation is counteracted. However, the known method has the disadvantage that it takes a comparatively long time to form a layer of oxide. In the manufacture of integrated circuits, this means either that the rate of passage of the wafers to be treated is low or that additional furnaces have to be arranged. For the formation of, for example, a layer of oxide having a thickness of 800 nm, a wafer would be treated for about 9 hours, about 1 hour of this treatment being a dry oxidation at 1000.degree. C. and about 8 hours being a wet oxidation at 920.degree. C. The time required to reduce the temperature in a furnace from 1000.degree. C. to 920.degree. C. then has not yet been taken into consideration.